1. Field of the Invention
The present invention relates to a propeller fan, and more particularly to vanes of propeller fan.
2. Background of the Invention
Generally, as shown in FIG. 3, a propeller fan has a plurality of vanes 4, each circumferentially spaced out at a predetermined distance and fixed at a hub 2 secured at a rotating axis (motor axis), having a predetermined length. The hub 2 is externally protruded with arms 6 to which the vanes 4 are attached by rivets 8. Each vane 4 has a cross-sectional shape like a curved plate from the leading part 10 to the trailing part 12 as illustrated in FIG. 4.
When the vanes 4 of the fan are rotated as per activation of a motor, a pressure difference is generated between the front side and the rear side of the fan, and air at the back side of the fan is discharged forward by the pressure difference. The vanes also serve to guide the floor of air discharged forward.
However, there is a problem in the vanes each having the cross-sectional shape illustrated in FIG. 4 according to the prior art in that an abnormal flow phenomenon is greatly generated where reverse flow of air occurs at or near surface of the vanes and noises are also created.
FIG. 5 is a graph for illustrating velocity distribution at the van of the fan having the cross-sectional shape of FIG. 4, where R defines a radial distance from a hub to a tip end of the vane along external radial direction of the hub, Vz is an air velocity at the surface of the vane, the symbols of rectangle (xe2x96xa1) at the curve of the graph is the leading edge of the vane, the triangle (xcex94) is the trailing edge of the vane and the circle ("ogr") represents a medium part between the leading edge and the trailing edge of the vane.
As illustrated in FIG. 5, the air is noticed to flow backward as the air nears the hub of the vane (Vz is negative number), where noise is measured at 89 dB(A).
When the air flows backward on the surface of the vane, flow loss is increased to decrease fan efficiency, thereby resulting in generation of abnormal noise.
The present invention is disclosed to solve the aforementioned problems and it is an object of the present invention to provide a propeller fan adapted to prevent generation of abnormal flow such as reverse flows and the like to thereby increase fan efficiency and to keep from generation of abnormal noise.
In accordance with the object of the present invention, there is provided a propeller fan, the fan including a plurality of vanes, each vane fixed at a hub secured at a rotary axle, having a predetermined length toward external radial direction thereof and circumferentially spaced out at a predetermined gap, wherein each van has a cross-sectional shape like a flat surface at an external side of a leading edge thereof while an external side of a trailing edge thereof is bent with a predetermined radius of curvature (Rc), where the radius of curvature (Rc) is preferred to satisfy the following formula;   Rc  =            (              0.55        ∼        0.60            )              R      L      
where, RL is a vane length measured toward external radial direction of hub and RL is preferred to satisfy the following formula:       R    L    =            1      2        ⁢                            2          ⁢          v          ⁢                      xe2x80x83                    ⁢                      R            e                          ω            
where, xcexd is coefficient of kinematic viscosity
Re is a critical Reynolds number and
xcfx89 is an angular velocity of fan, while an angle (xcex8) at bent region of the trailing edge is preferred to have 8xc2x0xcx9c18xc2x0.